Aluminum halides

The trihalides of aluminum offer another example of the dangers of assuming ionic character of solids that are formally derived from ions. Aqueous solutions of what we assume to be AlF₃, AlCl₃, AlBr₃, and AlI₃ all exhibit the normal properties ionic solutions (they are electrically conductive, for example), but the solids are quite different: the melting point of AlF₃ is 1290°C, suggesting that it is indeed ionic. But AlCl₃ melts at 192°C — hardly consistent with ionic bonding, and the other two halides are also rather low-melting. Structural studies show that when AlCl₃ vaporizes or dissolves in a non-polar solvent it forms a dimer Al₂Cl₆. The two other halides exist only as dimers in all states.

The structural formula of the Al₂Cl₆ molecule shows that the aluminum atoms are bonded to four chlorines, two of which are shared between the two metal atoms. The arrows represent coordinate covalent bonds in which the bonding electrons both come from the same atom (chlorine in this case.)

As shown at the right above, the aluminum atoms can be considered to be located at the centers of two tetrahedra that possess one edge in common.